Method for producing methacrylic acid

ABSTRACT

The present invention provides a method for producing methacrylic acid through gas-phase catalytic oxidation of methacrolein or a methacrolein-containing gas with molecular oxygen or a gas containing molecular oxygen by the use of a fixed-bed multitubular reactor comprising a plurality of reaction tubes each having a catalyst layer therein, wherein the above catalyst layer is divided in the direction of the tube axis of a reaction tube into two or more layers, to thereby provide a plurality of reaction zones, and the catalyst is caused to be present in the catalyst layer in such a way that a reaction load ratio CRc(i) per unit mass of the catalyst in each reaction zone becomes 0.8 to 1.0.

TECHNICAL FIELD

The present invention relates to a method for producing methacrylicacid.

The present application claims the priority of Japanese PatentApplication No. 2003-307770 filed on Aug. 29, 2003, the contents ofwhich are incorporated herein by reference.

BACKGROUND ART

As it is well-known, there have been many proposals concerning a methodfor producing methacrylic acid through the gas-phase catalytic oxidationof methacrolein by using a catalyst so far.

Heat accumulation takes place in the catalyst layer because thegas-phase catalytic oxidation is an exothermic reaction. A locally hightemperature zone resulting from the heat accumulation is called a hotspot, and when the temperature of this zone becomes higher than itneeds, an excessive oxidation reaction takes place so than the yield ofthe target product is lowered. Consequently, in the industrialenforcement of the aforementioned oxidation reaction, the temperaturecontrol of the hot spot is an important subject, and especially when theconcentration of methacrolein in the raw gas is raised to increase theproductivity, the temperature of the hot spot tends to become high sothat a large restriction is forced upon the reaction conditions in thepresent situation.

Accordingly, there have been several proposals concerning a method forsuppressing the temperature of the hot spot so far. For example, amethod of packing a plurality of catalysts having different activity ina plurality of reaction zones to order of raise the activity from theinlet part to the outlet part for the raw gas (Patent document 1), amethod of packing a catalyst which has a higher compositional ratio ofphosphorous and a lower compositional ratio of arsenic nearer to theoutlet side for the raw gas (Patent document 2), a method of packing acatalyst which contains a lesser amount of the characteristic elementsuch as potassium nearer to the outlet side for the raw gas (Patentdocument 3), a method of packing the catalyst wherein a catalyst layeris divided into a plurality of reaction zones, and the activity of thefirst reaction zone of the inlet side for the reaction gas is adjustedto be higher than that of the second: reaction zone, and the activity ofthe third reaction zone and the zones thereafter are adjusted to behigher in ascending order (Patent document 4), and the like areexemplified.

-   Patent Document 1: Japanese Patent Application, First Publication    No. Hei 4-210937-   Patent Document 2: Japanese Patent Application, First Publication    No. 2000-70721-   Patent Document 3: Japanese Patent Application, First Publication    No. 2003-171339-   Patent Document 4: Japanese Patent Application, First Publication    No. 2003-261501

These are the methods of suppressing the heat value of the reaction perunit volume by lowering the rate of reaction per unit volume at theinlet side for the raw gas in the catalyst layer of the reactor toresult in lowering the temperature of the hot spot. Therefore, these areeffective in improving the yield by suppressing the consecutiveoxidation reaction and in lengthening the catalyst life by reducing thethermal load.

However, these methods only paid attention to suppressing thetemperature of the hot spot so that the quantity of oxidation per unitmass of the catalyst in each reaction zone of the catalyst layer has notbeen controlled at all and consequently the distribution of the load ofthe oxidation reaction in the catalyst layer has not become uniform andit has been feared that a portion where the load of the oxidationreaction is high may occur. At this portion, the probability of theoccurrence of failure in the reoxidation of the catalyst became high, sothat deterioration of the catalyst was accelerated and it has been aconcern that the life of the catalyst layer as a whole may bedrastically shortened.

The present invention has been achieved by taking the above-mentionedproblems into consideration and has objects to provide a method forproducing methacrylic acid through the gas-phase catalytic oxidation ofmethacrolein with molecular oxygen in the presence of a solid oxidationcatalyst by the use of a fixed-bed tubular reactor, wherein the localdeterioration of the catalyst is suppressed and the catalyst is usedstably for a long time by not only suppressing the temperature of thehot spot but also making the load of the oxidation reaction in thecatalyst layer uniform, and to provide catalyst layers and a fixed-bedmultitubular reactor.

DISCLOSURE OF INVENTION

A method for producing methacrylic acid concerning a first embodiment ofthe present invention is characterized by a method for producingmethacrylic acid through the gas-phase catalytic oxidation ofmethacrolein or a methacrolein-containing gas with molecular oxygen or agas containing molecular oxygen by the use of a fixed-bed multitubularreactor comprising a plurality of reaction tubes each having a catalystlayer therein, wherein the above catalyst layer is divided in thedirection of the tube axis of the reaction tube into two or more layers,to thereby provide a plurality of reaction zones, and the catalyst ispacked in such a way that the reaction load ratio per unit mass of thecatalyst for each reaction zone falls within a fixed range.

The method for producing methacrylic acid concerning a second embodimentof the present invention is a method in which the packing conditioncontrolling the substantial catalyst-component mass per volume of eachreaction zone is determined by the method used in the first embodimentby using the fixed-bed multitubular reactor provided with the reactiontube in which the temperature distribution in the catalyst layer can bemeasured, and by using the condition thus determined, tee catalyst ispacked into the fixed-bed multitubular reactor having the reaction tubein which the temperature distribution in the catalyst layer is not orcannot be measured and methacrylic acid is produced through thegas-phase catalytic oxidation of methacrolein or amethacrolein-containing gas with molecular oxygen or a gas containingmolecular oxygen.

In the method for producing methacrylic acid of the present invention,the reaction temperature of the gas-phase catalytic oxidation may be 250to 350° C.

The aforementioned methacrolein-containing gas may contain 3 to 9% byvolume of methacrolein, 5 to 15% by volume of oxygen and 5 to 50% byvolume of water vapor and the space velocity of the aforementionedmethacrolein-containing gas may be 300 to 3000 hr⁻¹.

The maximum value among the distribution of the values obtained bysubtracting the aforementioned temperature of the heat transfer mediumfrom the aforementioned temperature in the catalyst layer (ΔT) in thedirection of the tube axis of the aforementioned reaction tube may be35° C. or less.

The aforementioned fixed-bed tubular reactor may be a multitubularreactor provided with a plurality of reaction tubes having an internaldiameter of 10 to 40 mm and a heat transfer medium bath.

According to the present invention, in the method for producingmethacrylic acid through the gas-phase catalytic oxidation ofmethacrolein with molecular oxygen in the presence of a solid oxidationcatalyst by the use of a fixed-bed tubular reactor, the localdeterioration of the catalyst can be suppressed and the catalyst can beused stably for a long time by not only suppressing the temperature ofthe hot spot but also making the load of the oxidation reaction in thecatalyst layer uniform.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a schematic diagram showing a longitudinal section of anexample of a fixed-bed multitubular reactor usable in the presentinvention.

FIG. 2 is a schematic diagram of a longitudinal section of the reactiontube of the aforementioned reactor.

BEST MODE FOR CARRYING OUT THE TRENTON

In the present invention, the reaction synthesizing methacrylic acid isperformed using a fixed-bed tubular reactor. The aforementionedfixed-bed tubular reactor is provided with a plurality of reaction tubeswhich are composed of cylinders, a catalyst to be set in each reactiontube to fill at least a part of the cross section of the reaction tube,a catalyst layer formed in the reaction tube including the range wherethe aforementioned catalyst is positioned, and a gas supplying devicewhich supplies methacrolein or a methacrolein-containing gas andmolecular oxygen or a gas containing molecular oxygen to theaforementioned reaction tubes.

FIG. 1 is a schematic diagram showing a longitudinal section of anexample of a fixed-bed multitubular reactor usable in the presentinvention The fixed-bed multitubular reactor has a cylindrical main body7, a hemispherical introduction part 8 and a hemispherical derivationpart 9 each being fixed at opposite ends of the main body 7 throughcircular tubesheets 10, 11. A raw gas inlet 1 is formed at theintroduction part 8 and a raw gas outlet 2 is formed at the derivationpart 9. Between the tubesheets 10, 11, many reaction tubes 5 are fixedparallel to the axis of the main body 7. Owing to this, the raw gassupplied from the raw gas inlet 1 is discharged from the raw gas outlet2 through the many reaction tubes 5. On the other hand, a heat transfermedium inlet 3 is equipped at one end of the main body 7 while a heattransfer medium outlet 4 is equipped at the other end and the heattransfer medium supplied from the heat transfer medium inlet 3 isdischarged from the heat transfer medium outlet 4 passing through themany reaction tubes 5.

The reaction tube has, as shown in FIG. 2, n layers of reaction zonesdivided in the direction of the tube axis, the number i (i is an integerfrom 1 to n) being set to each reaction zone in the sequential orderfrom the upstream side to the downstream side of the raw gas. In eachreaction zone, m temperature measuring points are set at intervals inthe direction of the axis, the distance between a j th temperaturemeasuring point and a j+1 th temperature measuring point in thesequential order from the upstream side to the downstream side of theraw gas in each reaction zone being represented by Li(j). Thetemperature of the j th and j+1 th measuring points of the i th reactionzone are represented as Ti(j) and Ti(j+1), respectively. The loadingweight of the catalyst of each reaction zone is represented as Wc(i) andthe temperature of the heat transfer medium existing among the reactiontubes is represented as TB.

The configurations of these fixed-bed tubular reactors are notparticularly limited, but from the industrial point of view, amultitubular reactor provided with thousands to tens of thousands ofreaction tubes having an internal diameter of 10 to 40 mm is preferableand a multitubular reactor provided with a heat transfer medium bath ispreferable. The heat transfer medium is not particularly limited, and afused-salt containing potassium nitrate, sodium nitrite and the like isexemplified.

A solid oxidation catalyst is used as the catalyst, and the catalyst isnot particularly limited as long as the solid catalyst for oxidation isused and a conventionally known catalyst such as a composite oxidecontaining molybdenum can be used, however, a composite oxiderepresented by the following compositional formula is preferable.Mo_(a)P_(b)Cu_(c)V_(d)X_(e)Y_(f)O_(g)(Wherein Mo, P, Cu, V and O represent molybdenum, phosphorous, copper,vanadium and oxygen, respectively; X represents at least one elementselected from the group consisting of iron, cobalt, nickel, zinc,magnesium, calcium, strontium, barium, titanium, chromium, tungsten,manganese, silver, boron, silicon, tin, lead, arsenic, antimony,bismuth, niobium, tantalum, zirconium, indium, sulfur, selenium,tellurium, lanthanum and cerium; Y represents at least one elementselected from the group consisting of potassium rubidium, cesium andthallium; subscripts a, b, c, d, e, f and g represent an atomic ratio ofeach element, respectively; when a is 12, b is in the range of from 0.1to 3, c is in the range of from 0.01 to 3, d is in the range of from0.01 to 3, e is in the range of from 0 to 10, f is in the range of from0.01 to 3 and g represents the atomic ratio of oxygen that fulfills therequirement of the valence of each element above.)

Further, the preparation method of the catalyst to be used is notparticularly limited, and conventionally various well-known methods canbe used provided that they don't cause undesirable maldistribution ofthe components. The raw materials to be used in the preparation of thecatalyst are not particularly limited either, and nitrates, carbonates,acetates, ammonium salts, oxides, halides and the like of each elementcan be used in combination. As a raw material for molybdenum, forexample, ammonium paramolybdate, molybdenum trioxide, molybdic acid,molybdenum chloride and the like can be used.

The catalyst to be used may be used without a carrier, however, asupported catalyst supported on an inactive carrier such as silica,alumina, silica-alumina, silicon carbide and the like or a catalystdiluted with these inactive carriers can be used.

The catalyst layer refers to a space portion where at least a catalystis contained in a reaction tube of the fixed-bed tubular reactor.Namely, it refers to not only the space where only the catalyst ispacked but also the space where the catalyst is diluted with theinactive carrier and the like. However, a space portion at each end pastof the reaction tube where nothing is packed or a space portion whereonly the inactive carrier is packed is not included in the catalystlayer because the catalyst is not substantially contained.

Now, the reaction for producing methacrylic acid through the gas-phasecatalytic oxidation of methacrolein with molecular oxygen in thepresence of a solid oxidation catalyst by the use of a fixed-bed tubularreactor is usually carried out under a reaction temperature in the rangeof from 250 to 350° C. At this time, the reaction raw material is notparticularly limited as long as it contains methacrolein and oxygen,however, generally a gas containing 3 to 9% by volume of methacrolein, 5to 15% by volume of oxygen and 5 to 50% by volume of water vapor(hereinafter merely referred to as a raw gas) is used.

The raw gas to be used here may contain a small amount of impuritieswhich don't exert any substantial influence on the present reaction suchas lower saturated aldehydes, ketones and the like, or may be diluted byadding an inert gas such as carbon dioxide and the like. The flow rateof the raw gas is not particularly limited, however, a flow rate whereinthe space velocity of the raw gas becomes 300 to 3000 hr⁻¹, especially500 to 2000 hr⁻¹, is preferable. The reaction temperature of theaforementioned oxidation reaction is preferably 250 to 350° C.,especially 260 to 330° C. The reaction pressure can be taken from anormal pressure to several atmospheric pressures.

In the practical enforcement of the present invention, use of the air asan oxygen source of the raw gas is economically advantageous.

When the raw gas is flowed through the catalyst layer kept at thereaction temperature of 250 to 350° C. in the reaction tube by the useof the aforementioned gas supplying device, an oxidation reaction iscarried out in the catalyst layer and methacrylic acid is mainlyproduced. On this occasion, a substantial amount of the catalystcomponent per unit volume of the inlet side of the fixed-bedmultitubular reactor is reduced to suppress the temperature of the hotspot in the inlet side of the catalyst layer.

As the method for reducing the substantial amount of the catalystcomponent per unit volume, so far a publicly known technology has beenused and, for example, 1) a method in which the catalyst layer isdivided into a plurality of reaction zones and the catalyst layer of thegas inlet side is diluted with an inert material, 2) a method it whichthe catalyst layer is divided into a plurality of reaction zones and asupported rate of the catalyst active material (a mass proportion of theactive material per one catalyst) is made larger in ascending order fromthe gas inlet side to the gas outlet side and 3) a method in which thecatalyst layer is divided into a plurality of reaction zones and a sizeof the catalyst molded article is made smaller in ascending order fromthe gas inlet side, to the gas outlet side are exemplified.

By carrying out such methods, the temperature of the hot spot inside thecatalyst layer is suppressed. As the scope of the definition that thetemperature of the hot spot is suppressed, though it differs dependingon the reaction system, in accordance with the investigation of thepresent inventors so far, in the case of producing methacrylic acidthrough a gas-phase oxidation using a solid catalyst such as these inthe present invention, the following is given, wherein the maximum valueamong the distribution of the values after subtracting the temperatureof the heat transfer medium from the temperature in the catalyst layer(ΔT) in the direction of the tube axis of the reaction tube is 35° C. orless. However, because of the foregoing reason, by this method alone itis apprehended that the distribution of the load of the oxidationreaction in the catalyst layer may become inhomogeneous and a high loadportion of the oxidation reaction way occur.

Consequently, by using the following method, for example, a substantialmass of the catalyst component per unit volume in the catalyst layer canbe optimized: 1) the catalyst layer is divided into a plurality ofreaction zones and a substantial mass of the catalyst component per unitvolume is controlled in each reaction zone by an optional method such asthose mentioned above; 2) the reaction is started using the catalystlayer thus obtained; 3) after the start of the reaction, the temperatureof the center of the catalyst layer in each reaction zone is measuredand from this result the reaction load ratio CRc(i) in each reactionzone is calculated; and 4) the operations 1) through 3) are repeated sothat the CRc(i) becomes in the range of from 0.8 to 1.0, preferably from0.9 to 1.0 in all reaction zones.

In these operations 1) through 4), the reaction load ratio CRc(i) ineach reaction zone is calculated in the following manner using thefollowing equations (1) to (3).

$\begin{matrix}{{{CRc}(i)} = {{{{Rc}(i)}/{Rc}}\;{MAX}}} & (1) \\{{{Rc}(i)} = {\left\lbrack {\sum\limits_{j = 1}^{m - 1}\frac{\left\{ {{\Delta\;{T_{i}(j)}} + {\Delta\;{T_{i}\left( {j + 1} \right)}}} \right\} \cdot {L_{i}(j)}}{2}} \right\rbrack/{{Wc}(i)}}} & (2) \\{{\Delta\;{T_{i}(j)}} = {{T_{i}(j)} - {TB}}} & (3)\end{matrix}$(wherein Rc(i) is the reaction load of the i th layer (° C.·m/kg), Li(j)is the distance (m) from the j th measuring point in the i th catalystlayer to the j+1 th measuring point in the direction of the tube axis,Wc(i) is the packing amount (kg) of the catalyst in the i th layer,Ti(j) is the catalyst-layer temperature (° C.) of the j th measuringpoint in the i th catalyst layer, TB is the heat transfer mediumtemperature (° C.), RcMAX is the maximum value of Rc(i), n is the numberof the reaction zones, and m is the number of the temperature measuringpoints in the i th catalyst layer. i is 1 to n. i and j are counted inturn from the inlet side of the reaction gas.)

In the calculation of the CRc(i) by the above-mentioned method, thereaction load is made uniform on the following assumptions: 1) the heatvalue of the catalyst layer is proportional to the quantity of theoxidation reaction in which the catalyst has taken part; 2) thetemperature distribution in the radial direction of the catalyst layerdoes not exist; 3) the overall heat transfer coefficient between theheat transfer medium and the catalyst layer is constant regardless ofthe reaction zones; 4) in each reaction zone, the heat brought in by thegas flowing in from the inlet part of the reaction zone or taken outfrom the outlet part of the reaction zone is not taken intoconsideration.

By assuming in this manner, the value obtained by subtracting thetemperature of the heat transfer medium from the temperature in thecatalyst layer (ΔT) and the quantity of the oxidation reaction in whichthe catalyst has taken part are expressed in a linear relationship,Consequently, by integrating the ΔT from the inlet part to the outletpart of each reaction zone along the direction of the tube axis anddividing the resultant value by the packed catalyst mass in the reactionzone, the load of the oxidation reaction in each reaction zone can becalculated.

On this occasion, depending on a position in the catalyst layer, becausethere is a slight difference in a proportion of occurrence of the mainreaction which produces methacrylic acid and the reaction which producesCOx or the other components; and there is more or less a temperaturedistribution in the radial direction of the catalyst layer, and further,there is a change in the overall heat transfer coefficient bycontrolling a substantial mass of the catalyst component in eachreaction zone, and there is a take out of a small portion of thegenerated heat in each reaction zone by the reaction gas to the outsideof the reaction zone, the above assumptions 1) through 4) do not holdtrue in the strict sense of the word However, in the method forproducing methacrylic acid using the catalyst and the fixed-bedmultitubular reactor of the present invention, it is essentiallysufficient to optimize the substantial mass of the catalyst componentusing the result calculated with the above method.

In the case of using the above method in the calculation of Rc(i), thenumber of divisions of the reaction zone can be optionally selected aslong as it is two or more and when increasing the number of divisions,the substantial mass of the catalyst component can be finely controlled.Consequently, the optimization can be performed more easily. However, incase that the number of divisions is increased to more than it needs, ittakes a long time to pack the catalyst so that the division of around 2to 4 is selected industrially.

The temperature in the catalyst layer can be measured by inserting andfixing many thermocouples in the reaction tube and also can be measuredby a thermocouple inserted in a protecting tube set up in the center ofthe cross section perpendicular to the tube axis direction of thereaction tube. On this occasion, it is preferable to have a structure,wherein the inside of the protecting tube is separated from the reactionsystem and the position of measuring the temperature can be changed byadjusting the length of insertion of the thermocouple.

Further, when measuring the temperate distribution of the catalyst layerof each reaction zone, precision of calculating the reaction load isimproved by taking a smaller value of the distance between thetemperature measuring points Li(j). However, a larger value of thedistance may be taken in the part where the temperature change in thetube axis direction of the catalyst layer is small. When determining thetemperature measuring point, both ends of the reaction zone have to bethe temperature measuring points and the difference between Ti(j) andTi(j+1) has to be 5° C. or less.

When occasion demands, there is a case that a substantial mass of thecatalyst component per unit mass of a catalyst molded article isdifferent by using properly a supported catalyst or a tableted catalystin each reaction zone. In this case, the value of not the mass of thepacked catalyst molded article but a calculated substantial muss of thecatalyst component in each reaction zone may be taken for Wc(i).

Now, in the present invention, a substantial mass of the catalystcomponent in each reaction zone is optimized by using the temperaturedistribution in the catalyst layer obtained by performing the reaction,however, in the case of an industrial fixed-bed multitubular reactor tobe used practically, there is a case that the temperature distributionof the catalyst layer cannot be measured in all the reaction tubes. Inthis case, for example, an optimization of a substantial mass of thecatalyst component in each reaction zone is carried out by using areproduced reaction tube as a mode) of the reaction tube composing thefixed-bed multitubular reactor, and by using the condition thusobtained, the catalyst may be packed in the reaction tube composing theindustrial fixed-bed multitubular reactor. Further, as a method of theoptimization, without packing the catalyst in the actual reaction tube,the following method may be use wherein the catalyst packing conditionis found so that CRc(i) in all the reaction zones comes in the abovespecified range using a simulator which predicts the temperaturedistribution in the catalyst layer (a simulation program using acomputer). In any case, in the present invention, it is necessary thatthe reaction load ratio [CRc(i)] per unit mass of the catalyst in eachreaction zone in the catalyst layer at the time of the reaction becomes0.8 g to 1.0, and the process of the optimization of the substantialmass of the catalyst component per unit volume in the catalyst layer isnot particularly limited. As mentioned above, by optimizing thesubstantial mass of the catalyst component tier unit volume in thecatalyst layer, the effect of the life extension of the presentinvention is sufficiently obtained even in the industrial fixed-bedmultitubular reactor.

Hereinafter, the method for producing methacrylic acid in the presentinvention will be entered into details with reference to the followingexamples.

The term “part” in the examples and in the comparative examples meanspart by mass. The catalyst component is obtained from the chargingamount of the raw materials. As a heat transfer medium of the fixed-bedmultitubular reactor, a fused-salt composed of 50% by mass of potassiumnitrate and 50% by mass of sodium nitrite was used. The temperature inthe catalyst layer is measured by a thermocouple inserted in aprotecting tube set up in the center of the cross section perpendicularto the tube axis direction of the reaction tube. At this time, theinside of the protecting tube is separated from the reaction system andthe position of measuring the temperature can be changed by adjustingthe length of insertion of the thermocouple.

The analysis of the raw gas and the gas produced by the reaction iscarried out by gas chromatography. The rate of reaction of methacrolein,the selectivity to the produced methacrylic acid and the yield ofmethacrylic acid are defined below, respectively.

-   The rate of reaction of methacrolein (%)=(B/A)×100-   The selectivity to methacrylic acid (%)=(C/B)×100-   The yield of methacrylic acid (%)=(C/A)×100

Wherein A is a number of moles of supplied methacrolein, B is a numberof moles of reacted methacrolein and C is a number of moles of producedmethacrylic acid.

EXAMPLE 1

In 300 parts of pure water, 100 parts of ammonium paramolybdate, 2.8parts of ammonium methavanadate and 9.2 parts of cesium nitrate aredissolved. To the resultant solution, a solution obtained by dissolving8.2 parts of 85 mass % phosphoric acid in 10 parts of pure water and asolution obtained by dissolving 1.1 parts of telluric acid in 10 partsof pure water are added while stirring and heated to 95° C. whilestirring. Then, a solution obtained by dissolving 3.4 parts of coppernitrate, 7.6 parts of ferric nitrate, 1.4 parts of zinc nitrate and 1.8parts of magnesium nitrate in 80 parts of pure water is added. Further,the mixed solution is sired at 100° C. for 15 minutes and a slurry thusobtained is dried by using a spray dryer.

To 100 parts of thus obtained dried material, 2 parts of graphite isadded and mixed and molded by a tableting machine into a pellet shedtablet which has an external diameter of 5 mm and a length of 5 mm. Theresultant tablet is calcined under airflow at 380° C. for 5 hours and acatalyst is obtained, The composition of the catalyst in atomic ratioexcept for oxygen is Mo₁₂ P_(1.5) Cu_(0.3) V_(0.5) Fe_(0.4) Te_(0.1)Mg_(0.15) Zn_(0.1) Cs₁.

A fixed-bed tubular reactor, equipped with a heat transfer medium bath,having an internal diameter of 25.4 mm made of SUS 304 is used for thereaction. The catalyst layer is divided into two reaction zones. In thegas inlet side reaction zone located at the supplying side of the rawgas, a mixture of 0.62 kg (520 mL) of the catalyst and 240 mL ofaluminum spheres having an external diameter of 5 mm is packed. Thelength of the catalyst layer of the raw gas inlet side reaction zone is1502 mm.

Then, 0.91 kg (760 mL) of the catalyst is packed in the gas outlet sidereaction zone. At this time, the length of the catalyst layer of the gasoutlet side reaction zone is 1505 mm.

Then, the heat transfer medium temperature is 312° C. and the raw gascomposed of 6.0% by volume of methacrolein, 10% by volume of oxygen, 10%by volume of water vapor and 74.0% by volume of nitrogen is introducedat a space velocity of 170 hr⁻¹.

When the temperature of the catalyst layer is measured two days afterthe start of the reaction, a hot spot having the maximum temperature atthe position 400 mm from the upstream end of the gas outlet sidereaction zone is observed and ΔT at the maximum temperature is 32° C.Rc(i) of the raw gas inlet side reaction zone is 52.6° C.·m/kg, CRc(i)is 1.0, Rc(i) of the raw gas outlet side reaction zone is 48.7° C.·m/kgand CRc(i) is 0.93. The rate of reaction of methacrolein is 83.60%, theselectivity to methacrylic acid is 84.4% and the yield of methacrylicacid is 70.6%. Even at 30 days after the start of the reaction, the rateof reaction of methacrolein 83.5% with the same heat transfer mediumtemperature of 312° C. so that the catalyst layer is keeping asufficient reaction activity.

COMPARATIVE EXAMPLE 1

The reaction is carried out in the same manner as in Example 1 exceptthat the packing amount of the catalyst in the gas inlet side reactionzone is increased and the heat transfer medium temperature is set to308° C. In the gas inlet side reaction zone, a mixture of 0.80 kg (670mL) of the catalyst and 90 mL of aluminum spheres having an externaldiameter of 5 mm is packed. The length of the catalyst layer of the gasinlet side reaction zone is 1498 mm.

Then, 0.91 kg (760 mL) of the catalyst is packed in the gas outlet sidereaction zone. At this time, the length of the catalyst layer of the gasoutlet side reaction zone is 1502 mm.

When the temperature of the catalyst layer is measured two days afterthe start of the reaction, a hot spot having the maximum temperature atthe position of 350 mm from the upstream end of the gas inlet sidereaction zone is observed and ΔT at the maximum temperature is 33° C.Rc(i) of the gas inlet side reaction zone is 54.5° C.·m/kg, CRc(i) is1.0, Rc(i) of the gas outlet side reaction zone is 41.5° C.·m/kg andCRc(i) is 0.76. The rate of reaction of methacrolein is 85.8%, theselectivity to methacrylic acid is 83.8% and the yield of methacrylicacid is 71.9%.

The catalyst layer packed in this condition shows a fast activitylowering and at 30 days after the start of the reaction, the rate ofreaction of methacrolein at the initial stage of the reaction can not bemaintained even with the increased heat transfer medium temperature of320° C. From this result, when the catalyst life is defined as theperiod in which the heat transfer medium temperature reaches 350° C. inthe operating procedure of maintaining a certain rate of reaction ofmethacrolein by raising the heat transfer medium temperature tocompensate the activity lowering caused by the catalyst deterioration,it is obvious that the catalyst life should become short in thiscomparative example though the substantial mass of the catalystcomponent in the catalyst layer is larger than that in Example 1.

EXAMPLE 2

The reaction is carried out in the same manner as in Example 1 exceptthat the thermocouple protecting tube is not inserted in the reactiontube and the temperature distribution in the catalyst layer during thereaction is not measured. The length of the catalyst layer of the gasinlet side reaction zone is 1495 mm. The length of the catalyst layer ofthe gas outlet side reaction zone is 1494 mm.

At the time two days after the start of the reaction, the rate ofreaction of methacrolein is 83.7%, the selectivity to methacrylic acidis 84.6% and the yield of methacrylic acid is 70.8%. Even at 30 daysafter the start of the reaction, the rate of reaction of methacrolein is83.6% with the same heat transfer medium temperate of 312° C. so thatthe catalyst layer is keeping a sufficient reaction activity.

EXAMPLE 3

The reaction is carried out in the same manner as in Example 1 exceptthat the catalyst layer is divided into three reaction zones and theheat transfer medium temperature and the catalyst packing condition ofeach reaction zone are changed. In the gas inlet side reaction zone, amixture of 0.40 kg (330 mL) of the catalyst and 180 mL of aluminumspheres having an external diameter of 5 mm is packed. The length of thecatalyst layer of the gas inlet side reaction zone is 1001 mm. Then, inthe intermediate reaction zone, a mixture of 0.50 kg (420 mL) of thecatalyst and 90 mL of aluminum spheres having an external diameter of 5mm is packed. The length of the catalyst layer of this reaction zone is1000 mm. Then, 0.60 kg (510 mL) of the catalyst is packed in the gasoutlet side reaction zone. The length of the catalyst layer of the gasoutlet side reaction zone is 1004 mm. The reaction is started with theheat transfer medium temperature of 313° C.

When the temperature of the catalyst layer is measured two days afterthe start of the reaction, a hot spot having the maximum temperature atthe position 200 mm from the upstream end of the gas outlet sidereaction zone is observed and ΔT at the maximum temperature is 30° C.Rc(i) of the gas inlet side reaction zone is 52.2° C.·m/kg, CRc(i) is1.0, Rc(i) of the intermediate reaction zone is 52.4° C.·m/kg, CRc(i) is1.0, Rc(i) of the raw gas outlet side reaction zone is 47.8° C.·m/kg andCRc(i) is 0.91. The rate of reaction of methacrolein is 83.4%, theselectivity to methacrylic acid is 85.1% and the yield of methacrylicacid is 70.90%. Even at 30 days after the start of the reaction, therate of reaction of methacrolein is 83.5% with the same heat transfermedium temperature of 313° C. so that the catalyst layer is keeping asufficient reaction activity.

So far, the method for producing methacrylic acid with respect to thepresent invention has been explained, however, the present invention isnot limited to the above embodiment and can be suitably modified as longas it does not deviate from the scope of the present invention.

INDUSTRIAL APPLICABILITY

According to the present invention, in the method for producingmethacrylic acid through the gas-phase catalytic oxidation ofmethacrolein with molecular oxygen in the presence of a solid oxidationcatalyst by the use of a fixed-bed tubular reactor, the localdeterioration of the catalyst can be suppressed and the catalyst can beused stably for a long time by not only suppressing the temperature ofthe hot spot but also making the load of the oxidation reaction in thecatalyst layer uniform.

1. A method for producing methacrylic acid through gas-phase catalyticoxidation of methacrolein or a methacrolein-containing gas withmolecular oxygen or a gas containing molecular oxygen by the use of afixed-bed multitubular reactor comprising a plurality of reaction tubeseach having a catalyst layer therein, wherein the above catalyst layeris divided in the direction of the tube axis of a reaction tube into twoor more layers, to thereby provide a plurality of reaction zones, andthe catalyst is caused to be present in the catalyst layer in such a waythat a reaction load ratio CRc(i) per unit mass of the catalyst in eachreaction zone defined by the following equations (1) to (3) becomes 0.8to 1.0, $\begin{matrix}{{{CRc}(i)} = {{{Rc}(i)}/{RcMAX}}} & (1) \\{{{Rc}(i)} = {\left\lbrack {\sum\limits_{j = 1}^{m - 1}\frac{\left\{ {{\Delta\;{T_{i}(j)}} + {\Delta\;{T_{i}\left( {j + 1} \right)}}} \right\} \cdot {L_{i}(j)}}{2}} \right\rbrack/{{Wc}(i)}}} & (2) \\{{\Delta\;{T_{i}(j)}} = {{T_{i}(j)} - {TB}}} & (3)\end{matrix}$ wherein Rc(i) is the reaction load of an i th layer (°C.·m/kg), Li(j) is a distance (m) from a j th measuring point in a i thcatalyst layer to the j+1 th measuring point in the direction of thetube axis, Wc(i) is a packing amount (kg) of the catalyst in the i thlayer, Ti(j) is a catalyst-layer temperature (° C.) of the j thmeasuring point in the i th catalyst layer, TB is a heat transfer mediumtemperature (° C.), RcMAX is a maximum value of Rc(i), n is the numberof the reaction zones, and m is the number of the temperature measuringpoints in the i th catalyst layer; i is 1 to n; i and j are counted inturn from the inlet side of the reaction gas.
 2. A method for producingmethacrylic acid, wherein a packing condition controlling a substantialmass of the catalyst component per the volume of each reaction zone isdetermined by the method of claim 1 by using the fixed-bed multitubularreactor provided with the reaction tube in which the temperaturedistribution in the catalyst layer can be measured, and by using thecondition thus determined, the catalyst is packed into the fixed-bedmultitubular reactor having the reaction tube in which the temperaturedistribution in the catalyst layer is not or cannot be measured and thereaction is performed.
 3. The method for producing methacrylic acidaccording to claim 1, wherein the reaction temperature of the gas-phasecatalytic oxidation is 250 to 350° C.
 4. The method for producingmethacrylic acid according to claim 1, wherein themethacrolein-containing gas contains 3 to 9% by volume of methacrolein,5 to 15% by volume of oxygen, and 5 to 50% by volume of water vapor, andthe space velocity of the methacrolein-containing gas is 300 to 3000hr⁻¹.
 5. The method for producing methacrylic acid according to claim 1,wherein the maximum value among the distribution of the values obtainedby subtracting the temperature of the heat transfer medium from thetemperature in the catalyst layer (ΔT) in the direction of the tube axisof the reaction tube is 35° C. or less.
 6. The method for producingmethacrylic acid according to claim 1, wherein a multitubular reactorprovided with a plurality of reaction tubes having an internal diameterof 10 to 40 mm and a heat transfer medium bath are used as the fixed-bedmultitubular reactor.